This invention pertains to FM (frequency modulation) receivers and other wireless receivers such as in combined radios. FM, for example, is popular in many developed countries and is growing in popularity in a number of developing countries.
In the United States and Europe, FM broadcast stations use a bandwidth of 200 KHz assigned to them at different frequencies or positions within the 87.5 MHz to 108 MHz frequency band. In Japan the FM band or available frequency spectrum is a 76 MHz to 90 MHz band, and an FM channel can be centered at multiples of 50 KHz, with a frequency spacing of at least 200 KHz between any two valid stations.
With the growing popularity of FM transmission in a number of developing countries, as well as the developed countries, low-cost integrated FM receivers have become important to integrate into mobile handsets like cell phones and Internet devices.
Bluetooth and/or WLAN (wireless local area network) activity can cause problematic audible artifacts during FM reception in some combo system on a chip units (SoCs) with FM, under a so-called “idle channel” condition. Suppose the FM carrier is kept unmodulated. Hence, silence is expected on demodulation. During BT/WLAN activity, two kinds of audible artifacts may show up: 1) audible “clicks” during start/stop of BT/WLAN activity, and 2) an audible “hum” at a frequency dependent on the BT/WLAN packet activity profile. Eliminating such artifacts is a desirable and important goal since audibility of spurious artifacts affects the user experience. Moreover, such audibility provides an important coexistence metric for FM performance testing that is readily measured.
One approach might reduce the audibility of the hum by improved de-coupling between the FM core and the aggressor cores or offending cores. However, such de-coupling likely would impose an increase in chip area. Another approach might add an inductor to an FM synthesizer power supply to improve co-existence performance. However, such inductor increases the number of parts in the system, such as on a printed circuit board, and likely imposes an additional cost.
Due to expected high adoption of low-cost integrated FM receivers in mobile handsets, inventive ways of delivering enhanced idle-channel coexistence and eliminating most or all such audible artifacts at low cost would be most desirable in this technology area.